The present invention relates to a method of making a master for manufacturing a stamper by transfer as well as a method of manufacturing an optical disc, in which when an optical disc is manufactured, an optical disc substrate having a concave and convex pattern of a groove for tracking, addressing or the like and a pit or the like of data recording by, for example, injection molding or 2P (Photo Polymerization) method is formed.
In recent years, the optical disc such as DVD (Digital Versatile Disc) has been in use as a recording medium in a wide range of fields.
The optical disc has the structure in which a minute concave and convex information pattern of grooves for providing various information signals, for example, an address signal and a tracking signal, pits as recording portions of data-information signal and the like are formed on an optically transparent optical-disc substrate made of polycarbonate and the like, on that pattern a reflective film made of a metal thin film such as aluminum film is formed, and further a protective film is formed thereon.
The optical disc is manufactured through the manufacturing process as shown in FIGS. 13A to 13J (See, for example, Japanese Laid-open Patent Application No. 2001-195791, Paragraphs [0002] to [0006]).
First, a glass substrate 90 is prepared (FIG. 13A) and a surface thereof is sufficiently smoothed, on which a resist layer 91 made of a light-sensitive photoresist (organic resist) is uniformly formed to make a resist substrate 92 (FIG. 13B).
Subsequently, while a laser beam for recording is made to scan spirally on the resist layer 91 of the resist substrate 92 from its inner circumference to its outer circumference or from its outer circumference to its inner circumference, the laser beam for recording on-off controlled correspondingly to an information signal pattern is applied to the resist layer 91 for forming an exposed master 93 which is exposed correspondingly to the concave and convex information pattern on the optical disc substrate to be ultimately obtained (FIG. 13C).
Next, by developing the resist layer 91, an original master 94 having a predetermined concave and convex pattern is obtained (FIG. 13D).
Next, a nickel-plated layer 95 is formed on the surface having a concave and convex pattern of the master 94 by electroforming (FIG. 13E). This plated layer 95 is exfoliated from the master 94 and is subjected to predetermined processing for providing a stamper 96 for molding, to which the concave and convex pattern of the master 94 is transferred (FIG. 13F).
By the injection molding method using this stamper for molding 96, an optical disc substrate 97 made of a polycarbonate of thermoplastic resin is molded (FIGS. 13G and 13H).
Subsequently, on the concave and convex surface of the optical disc substrate 97 are formed a reflective film 98 (FIG. 13I) of Al-alloy and a protective film 99 to obtain an optical disc 200 (FIG. 13J).
The optical disc manufactured in this way will be a product after quality examination. One of the quality items is asymmetry. The asymmetry indicates an asymmetrical characteristic of reproduced signal amplitude when a signal is reproduced, and becomes an important item as an index of quality of the signal reproduced from the optical disc and as a criterion for estimating a player and an optical pickup. Moreover, since the asymmetry is affected by dimensional variations of a concave portion (pit) among the concave and convex pattern formed on an optical disc, under circumstances where the concave and convex pattern becomes more microscopic as the latest optical disc becomes larger in capacity, the asymmetry has become a more important management item.
For the above reason, in order to suppress the dimensional variations of pits on an optical disc, the optimum manufacturing condition of each step is set in the above manufacturing process so that the asymmetry is managed so as to fall within a certain fixed range. Particularly, a process of making a master in manufacturing an optical disc is a process having a great effect on forming pits, for which a strict management is required.
Additionally, management range of the asymmetry is −5 to +15% under DVD-ROM standards.
However, since the asymmetry is recognized from a RF signal pattern when a signal is reproduced, it is difficult to measure the asymmetry from a latent image of an exposed resist layer, so that the asymmetry can be measured only from an optical disc at the stage of final product (FIG. 13J) in the above manufacturing process. Therefore, if the measured result is NG (No Good), a series of labor, manufacturing time and products so far will be wasted. Thus, in the case where malfunction occurs originating from manufacturing conditions in the exposing step, the loss will be significant.
Furthermore, since measurement results of the asymmetry proved after the final step is conventionally fed back to the manufacturing process, it is also impossible to correct promptly the manufacturing conditions. Particularly, as to the correction of manufacturing conditions in the exposing step, it takes plenty of time from a time point when the lot passes the exposing step until the corrected exposure condition based on the fed-back information from the final step of the lot is reflected. Furthermore, if the malfunction originating from manufacturing conditions occurs in the exposing step, it also takes time to investigate the cause of malfunction. Thus, it takes still more time to reflect the corrected condition with the result that the entire productivity is hindered.
Moreover, in the above exposing step, based on recording power of an exposing device, which is set correspondingly to a resist material forming the resist layer in a resist substrate, the resist layer is exposed under a fixed exposing condition. Thus, when recording sensitivity of the resist layer in the resist substrate changes, the change in sensitivity affects quality of recorded signals as it is. In addition, it is also difficult to deal with dispersion of the recording sensitivity among the lots of resist substrate.